This invention pertains to a novel process for the production of a fluoroelastomer; more particularly, it pertains to a suspension polymerization process for the production of a fluoroelastomer comprising copolymerized units of vinylidene fluoride, units of at least one other fluorinated major monomer and units of at least one cure site monomer and wherein said fluoroelastomer has substantially no ionic endgroups.
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF2) and units of at least one other copolymerizable fluorine-containing major monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether).
In order to develop the physical properties necessary for some end use applications, fluoroelastomers must be crosslinked. Typical curatives for promoting crosslinking include polyamines, polyols and the combination of an organic peroxide and a multifunctional unsaturated coagent. All these compounds form crosslinks by reacting with a cure site on the fluoroelastomer polymer chain. Examples of cure sites include a double bond, or a labile hydrogen, bromine, iodine, or chlorine atom. A common method of introducing a cure site into a fluoroelastomer made by continuous emulsion polymerization is to continuously add a minor amount of a copolymerizable cure site monomer, along with the major monomers (e.g. VF2, HFP, TFE, PAVE, etc.) to the polymerization reactor. In this manner, cure sites are randomly distributed along the resulting fluoroelastomer polymer chain. Suitable cure site monomers include bromine- or iodine-containing olefins, and bromine- or iodine-containing unsaturated ethers, non-conjugated dienes and 2-hydropentafluoropropylene (2-HPFP). Alternatively, or in addition to cure site monomers, cure sites may be introduced into the fluoroelastomer by conducting the polymerization in the presence of a chain transfer agent containing iodine, bromine or both. In this manner, a bromine or iodine atom is attached to the resulting fluoroelastomer polymer chain at one or both ends. Such chain transfer agents typically have the formula RIn, RBrn or RBrI, where R may be a C1-C3 hydrocarbon, a C1-C6 fluorohydrocarbon or chlorofluorohydrocarbon, or a C2-C8 perfluorocarbon, and n is 1 or 2.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. No. 4,214,060. Generally, fluoroelastomers are produced in an emulsion polymerization process wherein a water-soluble polymerization initiator and a relatively large amount of surfactant are employed. The resulting fluoroelastomer leaves the reactor in the form of a latex which must be degassed (i.e. freed from unreacted monomers), coagulated, filtered and washed. Emulsion processes suffer from several disadvantages including production of polymers having high Mooney viscosity, which tends to make it difficult to process these materials (i.e. mixing, extruding, molding) into cured articles, due to the presence of ionic endgroups on the fluoroelastomer polymer chains. Another disadvantage is that the polymer products contain impurities from retained surfactants, coagulants, buffers and defoamers. milliequivalents of ionic endgroups per kg fluoroelastomer. Ionic (or ionizable) endgroups include, but are not limited to, sulfate, sulfonate, sulfonic acid, carboxyl and carboxylate endgroups.
In particular, the present invention is directed to a suspension process for producing a fluoroelastomer having a selected molar ratio of copolymerized monomer units, said fluoroelastomer comprising copolymerized units of vinylidene fluoride major monomer, at least one other copolymerizable fluorinated major monomer, and at least one cure site monomer, comprising the steps of:
(A) charging a reactor with a quantity of an aqueous medium comprising a suspension stabilizer, said suspension stabilizer being present in said aqueous medium at a concentration of 0.001 to 3 parts by weight per 100 parts of said aqueous medium; said quantity of aqueous medium being such that a sufficient vapor space is left in said reactor for receiving gaseous monomer;
(B) charging the vapor space in said reactor with an initial quantity of a gaseous monomer mixture comprising vinylidene fluoride major monomer and at least one other fluorinated major monomer; and continuously mixing said aqueous medium and said monomer mixture to form a dispersion;
(C) initiating polymerization of said monomers at a temperature of 45xc2x0 C. to 70xc2x0 C. by adding to said dispersion an oil soluble organic peroxide polymerization initiator in an amount of 0.001 to 5 parts by weight per 100 parts of said aqueous medium, said initiator being added as a solution consisting essentially of 0.1 to 75 wt. % of an oil soluble organic peroxide in a water-soluble hydrocarbon solvent; and
(D) incrementally feeding to said reactor, during polymerization, so as to maintain a constant pressure in said reactor, said major monomers and at least one
On the other hand, in a suspension polymerization process, polymerization is carried out by dispersing one or more monomers, or an organic solvent with monomer dissolved therein, in water and using an oil-soluble organic peroxide. No surfactant or buffer is required and fluoroelastomer is produced in the form of polymer particles which may be directly filtered, i.e. without the need for coagulation, and then washed, thus producing a cleaner polymer than that resulting from an emulsion process. Also, the fluoroelastomer polymer chains are substantially free of ionic endgroups so that the Mooney viscosity is relatively low and the polymer has improved processability compared to polymer produced by an emulsion process (U.S. Pat. Nos. 3,801,552, 4,985,520 and 5,824,755).
A disadvantage of suspension polymerization processes disclosed in the prior art is that it is difficult to incorporate a cure site monomer uniformly into the polymer because polymerization rate and polymer molecular weight increase throughout the reaction period. Many cure site monomers, if present in excess, greatly hinder the polymerization reaction, so that the desired polymerization rate and polymer molecular weight can not be attained in suspension polymerization processes of the prior art.
In one aspect, the present invention provides a suspension polymerization process for the production of fluoroelastomers having uniformly distributed copolymerized units of one or more cure site monomers. The fluoroelastomers are characterized by having molecular weights sufficiently high to permit processing and curing using conventional techniques.
A further aspect of the invention relates to production of fluoroelastomer products which are substantially free of ionic endgroups. Such fluoroelastomers have lower Mooney viscosities than fluoroelastomers of similar comonomer composition and molecular weight produced from an emulsion polymerization process. By xe2x80x9csubstantially no ionic endgroupsxe2x80x9d is meant fewer than 1 cure site monomer, said major monomers and said cure site monomer being fed to the reactor in said selected molar ratio until a fluoroelastomer product having a number average molecular weight of between 50,000 to 2,000,000 daltons is obtained.
Optionally, a chain transfer agent may be added near the beginning of the polymerization process and additional quantities may be introduced throughout the process.
Another embodiment of this invention is the fluoroelastomer produced by the above process of this invention. Such a fluoroelastomer may be distinguished from a fluoroelastomer made by a different polymerization process in that fluoroelastomers of this invention i) are substantially free of ionic endgroups (as defined above), ii) contain polymer chain endgroups derived from an oil-soluble organic peroxide polymerization initiator, and iii) contain copolymerized units of a cure site monomer.
The present invention is directed to a suspension polymerization process for producing a fluoroelastomer which contains copolymerized units of vinylidene fluoride (VF2), units of at least one other fluorine-containing copolymerizable major monomer, and units of at least one cure site monomer. By xe2x80x9cmajor monomerxe2x80x9d is meant any copolymerizable monomer other than a cure site monomer. The resulting fluoroelastomer has a lower Mooney viscosity and fewer ionic endgroups than does a fluoroelastomer of the same monomer composition and molecular weight that is produced from an emulsion polymerization process. Fluoroelastomers produced by the suspension polymerization process of this invention have improved processability (i.e. improved extrudability, ease of mixing, moldability and demolding).
According to the present invention, fluorine-containing major monomers copolymerizable with VF2 include, but are not limited to, hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and a perfluoro(alkyl vinyl) ether (PAVE).
Perfluoro(alkyl vinyl ethers) (PAVE) suitable for use as monomers include those of the formula
CF2xe2x95x90CFO(Rfxe2x80x2O)n(Rfxe2x80x2O)mRfxe2x80x83xe2x80x83(I)
where Rfxe2x80x2and Rfxe2x80x3 are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A preferred class of perfluoro(alkyl vinyl ethers) includes compositions of the formula
CF2xe2x95x90CFO(CF2CFXO)nRfxe2x80x83xe2x80x83(II)
where X is F or CF3, n is 0-5, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A most preferred class of perfluoro(alkyl vinyl ethers) includes those ethers wherein n is 0 or 1 and Rf contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl ether) (PMVE) and perfluoro(propyl vinyl ether) (PPVE). Other useful monomers include compounds of the formula
CF2xe2x95x90CFO[(CF2)mCF2CFZO]nRfxe2x80x83xe2x80x83(III)
where Rf is a perfluoroalkyl group having 1-6 carbon atoms,
m=0 or 1, n=0-5, and Z=F or CF3.
Preferred members of this class are those in which Rf is C3F7, m=0, and n=1.
Additional perfluoro(alkyl vinyl ether) monomers include compounds of the formula
CF2xe2x95x90CFO[(CF2CF{CF3}O)n(CF2CF2CF2O)m(CF2)p]CxF2x+1xe2x80x83xe2x80x83(IV)
where m and n independently=0-10, p=0-3, and x=1-5.
Preferred members of this class include compounds where n=0-1, m=0-1, and x=1.
Additional examples of useful perfluoro(alkyl vinyl ethers) include
CF2=CFOCF2CF(CF3)O(CF2O)mCnF2n+1xe2x80x83xe2x80x83(V)
where n=1-5, m=1-3, and where, preferably, n=1.
PAVE-containing fluoroelastomers of the invention contain between 23 and 65 wt. % copolymerized VF2 units, preferably between 30 and 65 wt. % of such units. If less than 23 wt. % vinylidene fluoride units are present, the polymerization rate is very slow. In addition, good low temperature flexibility cannot be achieved. Vinylidene fluoride levels above 65 wt. % result in polymers that contain crystalline domains and are characterized by poor low temperature compression set resistance and reduced fluids resistance.
The PAVE content of the PAVE-containing fluoroelastomers of the invention ranges from 25 to 75 wt. %. If perfluoro(methyl vinyl ether) is used, then the fluoroelastomer preferably contains between 30 and 40 wt. % copolymerized PMVE units. If less than 25 wt. % perfluoro(alkyl vinyl ether) is present, the low temperature properties of the fluoroelastomers are adversely affected.
Copolymerized units of tetrafluoroethylene may also be present in the PAVE-containing fluoroelastomers of the invention at levels up to 30 wt. %. The presence of copolymerized units of TFE is desirable for the purpose of increasing fluorine content without unduly compromising low temperature flexibility. High fluorine content promotes good fluid resistance. If TFE is present as a monomer, it is preferably copolymerized in amounts of at least 3 wt. %. Levels of 3 wt. % or greater TFE lead to improved fluid resistance in some end use applications. TFE levels above 30 wt. % result in some polymer crystallinity which affects low temperature compression set and flexibility.
Fluoroelastomers containing units of PAVE are especially preferred in the present invention because of the combination of good low temperature sealing properties and good fluid resistance of these cured fluoroelastomers. Also, when 2-hydropentafluoropropylene cure site monomer is incorporated into PAVE-containing fluoroelastomers made by the suspension process of this invention, the fluoroelastomers show enhanced polyol curability compared to PAVE-containing fluoroelastomers made by an emulsion process.
The fluoroelastomers of the present invention also comprise units of one or more cure site monomers. Examples of suitable cure site monomers include: 2-hydropentafluoropropylene (2-HPFP, also referred to in the art as 1,1,3,3,3-pentafluoropropene); a non-conjugated diene (resulting in a reactive double bond cure site); a bromine- or iodine-containing olefin; and a bromine- or iodine-containing unsaturated ether. Units of cure site monomer are typically present in fluoroelastomers at a level of 0.3-7 wt. %, preferably 0.5-5 wt. % and most preferably between 0.7 and 3 wt %.
Brominated cure site monomers may contain other halogens, preferably fluorine. Examples of brominated olefin cure site monomers are bromotrifluoroethylene; 4-bromo-3,3,4,4-tetrafluorobutene-1(BTFB); and others such as vinyl bromide, 1-bromo-2,2-difluoroethylene; perfluoroalkyl bromide; 4-bromo-1,1,2-trifluorobutene; 4-bromo-1,1,3,3,4,4, -hexafluorobutene; 4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene; 6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1 and 3,3-difluoroallyl bromide. Brominated unsaturated ether cure site monomers useful in the invention include 2-bromo-perfluoroethyl perfluorovinyl ether and fluorinated compounds of the class CF2Brxe2x80x94Rfxe2x80x94Oxe2x80x94CFxe2x95x90CF2, such as CF2BrCF2Oxe2x80x94CFxe2x95x90CF2, and fluorovinyl ethers of the class ROCFxe2x95x90CFBr or ROCBrxe2x95x90CF2, where R is a lower alkyl group or fluoroalkyl group, such as CH3OCFxe2x95x90CFBr or CF3CH2OCFxe2x95x90CFBr.
Suitable iodinated cure site monomers include iodinated olefins of the formula: CHRxe2x95x90CHxe2x80x94Zxe2x80x94CH2CHRxe2x80x94I, wherein R is xe2x80x94H or xe2x80x94CH3; Z is a C1-C18 (per)fluoroalkylene radical, linear or branched, optionally containing one or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radical as disclosed in U.S. Pat. No. 5,674,959. Other examples of useful iodinated cure site monomers are unsaturated ethers of the formula: I(CH2CF2CF2)nOCFxe2x95x90CF2 and ICH2CF2O[CF(CF3)CF2O]nCFxe2x95x90CF2, and the like, wherein n=1-3, such as disclosed in U.S. Pat. No. 5,717,036. In addition, suitable iodinated cure site monomers including iodoethylene, 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); 3-chloro-4-iodo-3,4,4-trifluorobutene; 2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane; 2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene; 1,1,2,3,3,3 -hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethyl vinyl ether; 3,3,4,5,5,5-hexafluoro-4-iodopentene; and iodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyl iodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also useful cure site monomers.
Examples of non-conjugated diene cure site monomers include 1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene and others, such as those disclosed in Canadian Patent 2,067,891. A suitable triene is 8-methyl-4-ethylidene-1,7-octadiene.
Of the cure site monomers listed above, preferred compounds, for situations wherein the fluoroelastomer will be cured with peroxide, include 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); allyl iodide; and bromotrifluoroethylene. When the fluoroelastomer will be cured with a polyol, 2-HPFP is the preferred cure site monomer.
Additionally, iodine-containing endgroups, bromine-containing endgroups or mixtures thereof may optionally be present at one or both of the fluoroelastomer polymer chain ends as a result of the use of chain transfer or molecular weight regulating agents during preparation of the fluoroelastomers. The amount of chain transfer agent, when employed, is calculated to result in an iodine or bromine level in the fluoroelastomer in the range of 0.005-5 wt. %, preferably 0.05-3 wt. %.
Examples of chain transfer agents include iodine-containing compounds that result in incorporation of bound iodine at one or both ends of the polymer molecules. Methylene iodide; 1,4-diiodoperfluoro-n-butane; and 1,6-diiodo-3,3,4,4, tetrafluorohexane are representative of such agents. Other iodinated chain transfer agents include 1,3-diiodoperfluoropropane; 1,4-diiodoperfluorobutane; 1,6-diiodoperfluorohexane; 1,3-diiodo-2-chloroperfluoropropane; 1,2-di(iododifluoromethyl)-perfluorocyclobutane; monoiodoperfluoroethane; monoiodoperfluorobutane; 2-iodo-1-hydroperfluoroethane, etc. Particularly preferred are diiodinated chain transfer agents.
Examples of brominated chain transfer agents include 1-bromo-2-iodoperfluoroethane; 1-bromo-3-iodoperfluoropropane; 1-iodo-2-bromo-1,1-difluoroethane and others such as disclosed in U.S. Pat. No. 5,151,492.
Cure site monomers and chain transfer agents are typically added to the reactor as liquid solutions in the same solvent that is employed for the oil-soluble peroxide polymerization initiator (described below). In addition to being introduced into the reactor near the beginning of polymerization, quantities of chain transfer agent may be added throughout the entire polymerization reaction period, depending upon the desired composition of the fluoroelastomer being produced, the chain transfer agent being employed, and the total reaction time.
In the suspension polymerization process of this invention, (1) a gaseous monomer mixture of a desired composition (initial monomer charge) is introduced into the vapor space above an aqueous medium in a reactor. The aqueous medium comprises a suspension stabilizer in an amount of 0.001-3 parts (by weight) per 100 parts (by weight) of the aqueous medium. The monomer mixture is then dispersed in the aqueous medium and, optionally, a chain transfer agent is also added while the reaction mixture is agitated, typically by mechanical stirring. In the initial gaseous monomer charge, the relative amount of each monomer is dictated by reaction kinetics and is set so as to result in a fluoroelastomer having the desired (i.e. selected) molar ratio of copolymerized monomer units (e.g. very slow reacting monomers must be present in a higher molar amount relative to the other monomers than is desired in the composition of the fluoroelastomer to be produced); (2) the temperature of the reaction mixture is maintained in the range of 45xc2x0 C.-70xc2x0 C., preferably 50xc2x0 C.-60xc2x0 C.; (3) the suspension polymerization reaction is then initiated by adding an oil-soluble organic peroxide in an amount so as to result in between 0.001 and 5 parts by weight peroxide per 100 parts by weight of the aqueous medium. The peroxide is added as a solution consisting essentially of an oil-soluble organic peroxide in a water-soluble hydrocarbon solvent. Depending upon the nature of the fluoroelastomer to be produced and the total polymerization time, it may be necessary to add additional peroxide initiator to the reactor during the course of polymerization in order to keep the level of peroxide within the above range; and (4) additional quantities of the gaseous major monomers and cure site monomer (incremental feed) are added at a controlled rate throughout the polymerization in order to maintain a constant reactor pressure at a controlled temperature. Since polymerization rate constantly increases over the course of the reaction period, the flow rate of gaseous major monomer and cure site monomer must be increased over the course of the reaction period in order to maintain constant pressure within the reactor. The relative amount (i.e. molar ratio) of both the gaseous major monomers and cure site monomer in the incremental feed is approximately the same as the selected molar ratio of copolymerized monomer units in the fluoroelastomer to be prepared. The amount of polymer formed is approximately equal to the cumulative amount of incremental monomer feed. One skilled in the art will recognize that the molar ratio of monomers in the incremental feed is not necessarily exactly the same as that of the desired (i.e. selected) copolymerized monomer unit composition in the resulting fluoroelastomer because the composition of the initial charge may not be exactly that required for the selected final fluoroelastomer composition, or because a portion of the monomers in the incremental feed may dissolve into the polymer particles already formed, without reacting. However, in practice, the compositions of the initial charge and the incremental feed are often very similar to each other and to the composition of copolymerized monomer units desired in the fluoroelastomer to be produced. Polymerization times in the range of from 3 to 50 hours are employed in this invention.
The polymerization temperature is maintained in the range of 45xc2x0 C.-70xc2x0 C. If the temperature is below 45xc2x0 C., the rate of polymerization is too slow for efficient reaction on a commercial scale, while if the temperature is above 70xc2x0 C., suspended particles of the fluoroelastomer copolymer formed become sticky and are liable to cause plugging in the polymerization reactor and make it difficult to maintain a stable state of suspension during the polymerization reaction.
The polymerization pressure is in the range of 0.7 to 3.5 MPa, preferably 1.0 to 2.5 MPa. The desired polymerization pressure is initially achieved by adjusting the amount of gaseous monomers in the initial charge, and after the reaction is initiated, the pressure is adjusted by controlling the incremental gaseous monomer feed. The polymerization pressure is set in the above range because if it is below 0.7 MPa, the monomer concentration in the polymerization reaction system is too low to obtain a satisfactory reaction rate. In addition, the molecular weight does not increase sufficiently. If the pressure is above 3.5 MPa, the amount of monomer liquefied in the reactor is increased, thereby merely increasing the amount of monomer which is not consumed, resulting in poor production efficiency.
It is very important that cure site monomers, other than 2Hxe2x80x94PFP, not be present prior to initiation of the polymerization reaction. It is also important that cure site monomer introduced in the incremental feed not be added to the reactor in an excess molar amount. Otherwise, polymerization is terminated early and only low molecular weight fluoroelastomers are produced. The fluoroelastomers produced by this invention have number average molecular weights in the range of about 50,000 to 2,000,000 daltons. To obtain fluoroelastomers of such molecular weights having uniform composition of copolymerized units of major monomers and cure site monomer, cure site monomer must be added with the incremental feed in a set ratio equal to the selected level to be incorporated into the polymer. Cure site comonomer addition is controlled such that the ratio of cure site monomer to total incremental monomer feed is in the range of 0.3 wt. % to 7 wt. %, preferably in the range of 0.5 wt. % to 5 wt. %.
It is relatively easy to control the flow rate of incremental feed gaseous major monomers in order to maintain constant pressure within the reactor throughout the entire polymerization reaction period. However, controlling the flow rate of the liquid cure site monomer solution may be problematic. In the case of the gaseous major monomers, a pressure controller may simply increase the flow rate of gaseous monomer to the reactor in order to maintain constant pressure within the reactor as polymerization rate increases. In the early stages of the polymerization reaction, when the polymerization rate is low, and flow rate of gaseous major monomer incremental feed is very small, it may be necessary to use a gas accumulator between the reactor and major monomer source in order to accurately control the flow rate and thus maintain constant pressure within the reactor.
The flow rate of gaseous major monomer must be maintained in a set proportion to the flow rate of liquid cure site monomer solution throughout the entire reaction in order to produce a fluoroelastomer of uniform composition having the selected molar ratios of copolymerized units of major monomers and cure site monomer. Therefore as the flow rate of gaseous major monomers is increased during the reaction period, the flow rate of the liquid cure site monomer solution must be simultaneously increased by a proportional amount. One skilled in the art will readily recognize several means to accomplish this. For example, a flow rate monitor may be placed in the gaseous monomer incremental feed line and then the flow rate of the liquid cure site monomer can be increased proportionally, either manually or automatically, as the flow rate of gaseous monomer is increased. Alternatively, the average flow rate of gaseous major monomer may be determined over several discrete time periods throughout the total reaction time. The flow rate of the cure site monomer can then be set proportionally to the average gaseous monomer flow rate during each discrete time period.
The amount of fluoroelastomer copolymer formed is approximately equal to the amount of incremental feed charged, and is in the range of 10-300 parts by weight of copolymer per 100 parts by weight of aqueous medium, preferably in the range of 20-250 parts by weight of the copolymer.
The degree of copolymer formation is set in the above range because if it is less than 10 parts by weight, productivity is undesirably low, while if it is above 300 parts by weight, the solids content becomes too high for satisfactory stirring.
Oil-soluble organic peroxides which may be used to initiate polymerization in this invention include, for example, dialkylperoxydicarbonates, such as diisopropylperoxydicarbonate (IPP), di-sec-butylperoxydicarbonate, di-sec-hexylperoxydicarbonate, di-n-propylperoxydicarbonate, and di-n-butyl peroxydicarbonate; peroxyesters, such as tert-butylperoxyisobutyrate and tert-butylperoxypivalate; diacylperoxides, such as dipropionyl peroxide; and di(perfluoroacyl)peroxides or di(chlorofluoroacyl)peroxides such as di(perfluoropropionyl)peroxide and di(trichloro-octafluorohexanoyl)peroxide.
The use of dialkylperoxydicarbonates is preferable, and the use of IPP is most preferred. These oil soluble organic peroxides may be used alone or as a mixture of two or more types. The amount to be used is selected generally in the range of 0.001-5 parts by weight per 100 parts by weight of the aqueous medium, preferably 0.01-3 parts by weight. During polymerization some of the fluoroelastomer polymer chain ends are capped with fragments generated by the decomposition of these peroxides.
In the suspension polymerization process of the invention, the oil-soluble organic peroxide is added to the reactor as a solution consisting essentially of 0.1-75 wt % (preferably 1-60 wt. %) peroxide in a water-soluble hydrocarbon solvent. If the concentration of peroxide is over 75 wt %, the organic peroxide concentration is too high for safe transportation. On the other hand, if it is below 0.1 wt %, the concentration is so low that the amount of solvent to be recovered after polymerization becomes undesirably high.
The water-soluble hydrocarbon solvent contains no halogen atoms and is represented by the general formulas R1OH, R2COOR1, or R1COR3, where R1 and R3 are methyl or t-butyl groups, and R2 is hydrogen, a methyl group or a t-butyl group. The hydrocarbon solvents useful in the present invention do not have substantial adverse effects on the polymerization reaction because the chain transfer reactivity of these hydrocarbon solvents is relatively small. At the same time, they are soluble in the aqueous reactor medium. Further, only small amounts are contained in droplets comprised of the monomers and oil soluble organic peroxide in which the polymerization reaction occurs. Also, polymerization conditions are set so that both solvent and monomer concentrations are low (generally less that 10 wt. %) in the fluoroelastomer copolymer formed in the reactor. Thus recovery of the solvent and monomers is not difficult.
Specific examples of water-soluble, non-halogenated hydrocarbon solvents useful in this invention are methanol, tert-butyl alcohol, methyl formate, tert-butyl formate, methyl acetate, tert-butyl acetate, methyl pivalate, tert-butyl pivalate, acetone, methyl tert-butyl ketone, and di-tert-butyl ketone. The use of methanol, tert-butyl alcohol, methyl acetate, or tert-butyl acetate is preferable. Methyl acetate or tert-butyl acetate are most preferred. These solvents may be used alone or as a combination of two or more types.
Suspension stabilizers useful in the present invention include, for example, methyl cellulose, carboxymethyl cellulose, bentonite, talc, and diatomaceous earth. Methyl cellulose is preferred. Typically the number average molecular weight of the methyl cellulose is between 15,000 and 70,000. These suspension stabilizers may be used alone or as a combination of two or more types. The amount utilized is generally in the range of 0.001-3 parts by weight, preferably 0.01-1 part by weight per 100 parts by weight of the aqueous medium.
The monomer composition of the initial charge and that of the incremental feed are determined by gas chromatography. The monomer composition (i.e. the mole percentage of copolymerized monomer units) in the fluoroelastomer copolymer prepared is determined by dissolving the fluoroelastomer in deutero-acetone and carrying out 1H and 19F-NMR analysis, or by FTIR analysis of thin films. X-ray fluorescence is used to determine concentrations of bromine- and iodine-containing cure sites.
Another embodiment of this invention is the novel fluoroelastomers produced by the above suspension process of this invention. Such fluoroelastomers may be distinguished from fluoroelastomers made by a different polymerization process in that fluoroelastomers of this invention i) are substantially free of ionic endgroups (as defined above in the Summary of the Invention), ii) contain polymer chain endgroups derived from an oil-soluble organic peroxide polymerization initiator, and iii) contain copolymerized units of a cure site monomer. Emulsion-produced fluoroelastomers of similar copolymerized monomer unit composition as the fluoroelastomers of this invention will either have more than one milliequivalent of ionic endgroups per kg of fluoroelastomer, or the emulsion-produced fluoroelastomer will have endgroups derived from a water-soluble inorganic peroxide polymerization initiator (such as ammonium persulfate), or both.
The fluoroelastomers prepared by this invention are generally molded and vulcanized during fabrication into finished products such as seals, wire coatings, hose, etc. Suitable vulcanization methods employ polyol, polyamine, or organic peroxide compounds as curatives. Vulcanization with a polyol compound is especially advantageous because compression set resistance of the cured fluoroelastomer is generally better than that obtained using polyamine or peroxide curatives. Polyol curatives are particularly effective curatives for fluoroelastomers which contain copolymerized units of 2-HPFP cure site monomer, in particular fluoroelastomers comprising copolymerized units of 30-65 wt. % VF2, 30-40 wt. % PMVE, 3-30 wt. % TFE and 0.5-3 wt. % 2-HPFP.
When peroxide curatives are used, resistance of the cured fluoroelastomers to chemicals such as acids or bases is markedly improved. Peroxide curatives are particularly useful for vulcanizing fluoroelastomers which contain either a bromine-, or iodine-containing cure site monomer. The latter type of fluoroelastomers which also have iodine or bromine at one or more polymer chain ends cure especially well with peroxide curatives. PMVE-containing fluoroelastomers comprising copolymerized units of 30-65 wt. % VF2, 30-40 wt. % PMVE, 3-30 wt. % TFE and 0.5-3 wt. % of either BTFB, ITFB or allyl iodide are preferred peroxide-curable polymers. Fluoroelastomers containing copolymerized units of 30-65 wt. % VF2, 25-40 wt. % HFP, 3-30 wt. % TFE and 0.5-3 wt. % of either BTFB, ITFB or allyl iodide are also preferred peroxide-curable polymers.
Any of the known polyol aromatic crosslinking agents that require accelerators for satisfactory cure rates are suitable for use with the fluoroelastomers prepared by the present invention. The crosslinking agent is usually added in amounts of from about 0.5-4 parts by weight per hundred parts by weight fluoroelastomer (phr), usually 1-2.5 phr. Preferred crosslinking agents are di- tri-, tetrahydroxybenzenes, naphthalenes, anthracenes and bisphenols of the formula 
where A is a stable divalent radical, such as a difunctional aliphatic, cycloaliphatic, or aromatic radical of 1-13 carbon atoms, or a thio, oxy, carbonyl, sulfinyl, or sulfonyl radical; A is optionally substituted with at least one chlorine or fluorine atom; x is 0 or 1; n is 1 or 2 and any aromatic ring of the polyhydroxylic compound is optionally substituted with at least one atom of chlorine, fluorine, or bromine, a xe2x80x94CHO group, or a carboxyl or acyl radical (e.g. a xe2x80x94COR where R is OH or a C1-C8 alkyl, aryl, or cycloalkyl group). It will be understood from the above formula describing bisphenols that the xe2x80x94OH groups can be attached in any position (other than number one) in either ring. Blends of two or more such compounds can also be used.
Referring to the bisphenol formula shown in the previous paragraph, when A is alkylene, it can be, for example, methylene, ethylene, chloroethylene, fluoroethylene, difluoroethylene, 1,3-propylene, 1,2-propylene, tetramethylene, chlorotetramethylene, fluorotetramethylene, trifluorotetramethylene, 2-methyl-1,3-propylene, 2-methyl-1,2-propylene, pentamethylene, and hexamethylene. When A is alkylidene, it can be for example ethylidene, dichloroethylidene, difluoroethylidene, propylidene, isopropylidene, trifluoroisopropylidene, hexafluoroisopropylidene, butylidene, heptachlorobutylidene, heptafluorobutylidene, pentylidene, hexylidene, and 1,1 -cyclohexylidene. When A is a cycloalkylene radical, it can be for example 1,4-cyclohexylene; 2-chloro-1,4-cyclohexylene; 2-fluoro-1,4-cyclohexylene; 1,3-cyclohexylene; cyclopentylene; chlorocyclopentylene; fluorocyclopentylene; and cycloheptylene. Further, A can be an arylene radical such as m-phenylene; p-phenylene; 2-chloro-1,4-phenylene; 2-fluoro-1,4-phenylene; o-phenylene; methylphenylene; dimethylphenylene; trimethylphenylene; tetramethylphenylene; 1,4-naphthylene; 3-fluoro-1,4-naphthylene; 5-chloro-1,4-naphthylene; 1,5-naphthylene; and 2,6-naphthylene.
Other useful crosslinking agents include hydroquinone, dihydroxybenzenes such as catechol, resorcinol, 2-methyl resorcinol, 5-methyl resorcinol, 2-methyl hydroquinone, 2,5-dimethyl hydroquinone; 2-t-butyl hydroquinone; and 1,5-dihydroxynaphthalene.
Additional polyol curing agents include alkali metal salts of bisphenol anions, quaternary ammonium salts of bisphenol anions and quaternary phosphonium salts of bisphenol anions. For example, the salts of bisphenol A and bisphenol AF. Specific examples include the disodium salt of bisphenol AF, the dipotassium salt of bisphenol AF, the monosodium monopotassium salt of bisphenol AF and the benzyltriphenylphosphonium salt of bisphenol AF. Quaternary ammonium and phosphonium salts of bisphenol anions and their preparation are discussed in U.S. Pat. Nos. 4,957,975 and 5,648,429.
In addition, derivatized polyol compounds, such as diesters, are useful crosslinking agents. Examples of such compositions include diesters of phenols, such as the diacetate of bisphenol AF, the diacetate of sulfonyl diphenol, and the diacetate of hydroquinone.
When cured with polyol compounds, the curable compositions will also generally include a cure accelerator. The most useful accelerators are quaternary phosphonium salts, quaternary alkylammonium salts, or tertiary sulfonium salts. Particularly preferred accelerators are n-tetrabutylammonium hydrogen sulfate, tributylallylphosphonium chloride and benzyltriphenylphosphonium chloride. Other useful accelerators include those described in U.S. Pat. Nos. 5,591,804; 4,912,171; 4,882,390; 4,259,463 and 4,250,278 such as tributylbenzylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium chloride, benzyl tris(dimethylamino)phosphonium chloride; 8-benzyl-1,8-diazabicyclo[5,4,0]-7-undecenonium chloride, [(C6H5)2S+(C6H13)][Cl]xe2x88x92, and [(C6H13)2S(C6H5)]+[CH3CO2]xe2x88x92. In general, about 0.2 phr accelerator is an effective amount, and preferably about 0.35-1.5 phr is used.
If quaternary ammonium or phosphonium salts of bisphenols are used as curing agents, then addition of a cure accelerator is not necessary.
The polyol cure system will also contain a metal compound composed of a divalent metal oxide, such as magnesium oxide, zinc oxide, calcium oxide, or lead oxide, or a divalent metal hydroxide; or a mixture of the oxide and/or hydroxide with a metal salt of a weak acid, for example a mixture containing about 1-70 percent by weight of the metal salt. Among the useful metal salts of weak acids are barium, sodium, potassium, lead, and calcium stearates, benzoates, carbonates, oxalates, and phosphites. The amount of the metal compound added is generally about 1-15 phr, about 2-10 parts being preferred.
Polyamines and diamine carbamates are also useful curing agents for the compositions of the invention. Examples of useful polyamines include N,Nxe2x80x2-dicinnamylidene-1,6-hexanediamine, trimethylenediamine, cinnamylidene trimethylenediamine, cinnamylidene ethylenediamine, and cinnamylidene hexamethylenediamine. Examples of useful carbamates are hexamethylenediamine carbamate, bis(4-aminocyclohexyl)methane carbamate, 1,3-diaminopropane monocarbamate, ethylenediamine carbamate and trimethylenediamine carbamate. Usually about 0.1-5 phr of the carbamate is used.
The peroxide vulcanization method can be exemplified as follows. To a fluoroelastomer prepared by this invention is added (a) an organic peroxide, (b) a polyfunctional unsaturated compound, and (c) a divalent metal hydroxide, a divalent metal oxide, or a combination of the two.
Organic peroxides suitable for use include: 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane; 1,1 -bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane; n-butyl-4,4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane; 2,5-dimethylhexane-2,5 -dihydroxyperoxide; di-t-butyl peroxide; t-butylcumyl peroxide; dicumyl peroxide; alpha, alphaxe2x80x2-bis(t-butylperoxy-m-isopropyl)benzene; 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; 2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide, t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane; t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferred examples of the component organic peroxides include 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and alpha, alphaxe2x80x2-bis(t-butylperoxy-m-isopropyl)benzene. The amount compounded is generally in the range of 0.05-5 parts by weight, preferably in the range of 0.1-3 parts by weight per 100 parts by weight of the fluoroelastomer. This particular range is selected because if the peroxide is present in an amount of less than 0.05 parts by weight, the vulcanization rate is insufficient and causes poor mold release. On the other hand, if the peroxide is present in amounts of greater than 5 parts by weight, the compression set of the cured polymer becomes unacceptably high. In addition, the organic peroxides may be used singly or in combinations of two or more types.
Specific examples of the polyfunctional unsaturated compound used in the peroxide vulcanization method are triallyl cyanurate, trimethacryl isocyanurate, triallyl isocyanurate, trimethallyl isocyanurate, triacryl formal, triallyl trimellitate, N,Nxe2x80x2-m-phenylene bismaleimide, diallyl phthalate, tetraallylterephthalamide, tri(diallylamine)-s-triazine, triallyl phosphite, and N,N-diallylacrylamide. The amount compounded is generally in the range of 0.1-10 parts by weight per 100 parts by weight of the fluoroelastomer. This particular concentration range is selected because if the unsaturated compound is present in amounts less than 0.1 part by weight, crosslink density of the cured polymer is unacceptable. On the other hand, if the unsaturated compound is present in amounts above 10 parts by weight, it blooms to the surface during molding, resulting in poor mold release characteristics. The preferable range of unsaturated compound is 0.2-6 parts by weight per 100 parts fluoroelastomer. The unsaturated compounds may be used singly or as a combination of two or more types.
In addition, if necessary, other components, for example, fillers such as carbon black, Austin black, graphite, thermoplastic fluoropolymer micropowders, silica, clay, diatomaceous earth, talc, wollastonite, calcium carbonate, calcium silicate, calcium fluoride, and barium sulfate; processing aides such as higher fatty acid esters, fatty acid calcium salts, fatty acidamides (e.g. erucamide), low molecular weight polyethylene, silicone oil, silicone grease, stearic acid, sodium stearate, calcium stearate, magnesium stearate, aluminum stearate, and zinc stearate; coloring agents such as titanium white and iron red may be used as compounding additives. The amount of such filler compounded is generally in the range of 0.1-100 parts by weight, preferably 1-60 parts by weight, per 100 parts by weight of the fluoroelastomer. This range is selected because if the filler is present in amounts of less than 0.1 part by weight, there is little or no effect, while, on the other hand, if greater than 100 parts by weight are used, elasticity is sacrificed. The amount of processing aid compounded is generally less than 10 parts by weight, preferably less than 5 parts by weight, per 100 parts by weight of the fluoroelastomer. If the amount used is above the limit, heat resistance is adversely affected. The amount of a coloring agent compounded is generally less than 50 parts by weight, preferably less than 30 parts by weight per 100 parts by weight of the fluoroelastomer. If greater than 50 parts by weight is used, compression set suffers.
The fluoroelastomers prepared by the process of this invention are useful in many industrial applications including seals, wire coatings, tubing and laminates.